Electrostatics Charges at rest: Electrostatics
The part of physics which deals with the study of electrical charges at rest and its effect such as electrical forces, electric fields and electric potentials arising from static charges is called Electrostatics
Charge Charge : Additional property of Electron and Proton
Charge is an intrinsic or basic property of an elementary or subatomic particles due to which it produces electric and magnetic effects such as attraction, repulsion, electromagnetic induction current etc. charge is carried by electrons and protons in equal amount opposite in nature/character.
Charge on electron = - 1.6 x 10-19 Coulomb
Charge on proton = +1.6 x 10-19 Coulomb
| Charge on electron | = | charge on proton | = 1.6 x 10-19 Coulomb
Fig : electric field lines around a pair of point charges:
Unit of Charge
(1) S.I. unit : unit of electric charge is coulomb in S.I. system.
(2) c.g.s. unit : unit of electric charge is stat coulomb in c.g.s. system.
1 coulomb = 3 x 109 stat-coulomb
Basic Properties of Electric Charges
1. There exists only two types of charges, namely positive and negative.
2. Like charges repel and unlike charges attract each other.
3. Charge is a scalar quantity.
4. Charge is additive in nature.
5. Charge is quantized.
6. Charge is conserved.
7. Charge is not affected by its motion.
Conservation of charges:
Total charge in an isolated system is always conserved. When there are many bodies in an isolated system, the charges get transferred from one body to another but the net charge of the system remains same.
During rubbing or natural forces, no new charge is created. The charges are either redistributed or a neutron breaks up into proton and electron of equal and opposite charge.
main points of Conservation of charges are :
Charge of an isolated system is conserved.
Charge can not be created or destroyed but charged particles can be created or destroyed.
Quantization of charge
All free charges are integral multiples of a unit of charge e, where e = -1.602 × 10 -19 C i. e., charge on an electron or proton.
Thus charge q on
a body is always denoted by
q = ne
where n = any integer positive or negative
Point Charge Point Charge: Zero Size Structure
Point charges
Charges whose sizes are very small compared to the distance between them are called point charges
Stationary charge
If charge is not moving then it is said to be stationary charge. A stationary charged particle does not interact with a static magnetic field. There is no current formed by stationary charge.
Methods of Charging
There are three methods of charging a body :
1. Charging by Friction or rubbing.
2. Charging by conduction or contact
3. Charging by Induction
Comparison of Charge And Mass
|
CHARGE |
MASS |
|
Electric charge on the body may be positive or negative. |
Mass of a body is a positive quantity |
|
Charge carried by the body does not depend upon the velocity. |
Charge carried by the body depends upon the velocity. |
|
Charge is quantized |
Mass is not quantized yet. |
|
Electric charge is always conserved |
Mass can be changed into energy, it is not conserved. |
|
Force may be attractive or repulsive, according to the charge |
Gravitational force between two masses is always attractive. |
Coulomb's law
Coulomb's law states that “The magnitude of the electrostatic force of attraction or repulsion between two stationary electrical point charges is directly proportional to the product of the magnitudes of charges and inversely proportional to the square of the distance between them”. The force is along the straight line joining them. The electrostatic (coulombian) forces are conservative forces.
Let two stationary point charges q1 and q2 be located at A and B at a distance r,
Then , force between two charges , F,
F α q1 q2 …………………………………………………….…. (1)
F α 1/ r 2 ………………………………………………….……..(2)
Combining (1) and (2)
F α q1 q2 / r 2
F = k q1 q2 / r 2
Where k is a constant. Its value depends on nature of medium in which two charges are located and on the system of units used to measure F , q1 , q2 and r.
In S.I. units when two charges are located in vacuum or air, k = 1/4πϵ0 = 9 x 109
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